Hybrid cables are used for transmitting data in the oil and gas well survey. The excellent performance of optical fiber, such as anti-electromagnetic interference, small diameter, light weight, softness, high temperature resistance and corrosion resistance, etc., enable detectors with optical fiber sensing system to overcome the harsh environment in the oil well and measure environmental parameters accurately. Moreover, a detector of optical fiber sensing system with a small cross-sectional area is convenient to use in the well. When in use, the hybrid cable is placed in the well to collect sample data, and the detector receives optical signals and electricity through the hybrid cable. After the survey is completed, the cable is pulled out of the well. The hybrid cable can also be used for other downhole operations.
Design factors of hybrid cables for harsh environment of high pressure and high chemical corrosion (over 200° C., 100 MPa) include, but not limited to, selection of special optical fibers of corresponding temperature grades, and structure design of cables for protecting optical fibers. Existing hybrid cables generally have a number of fiber in the metal tube (FIMT) twisted with a number of circular insulated wires concentrically placed in the outer steel tube of the hybrid cable. The wall thickness of the outer steel tube is designed according to the downhole pressure requirement. Due to limitations on the outer diameter of the hybrid cable, the inner diameter of the steel tube in FIMT is also a fixed value when the outer diameter is constant. Since the thermal expansion coefficient of the metal is much higher than the thermal expansion coefficient of the optical fiber, the steel tube in FIMT becomes elongated when the FIMT is heated. If the optical fiber does not have sufficient excess length, the optical fiber will be stretched by the steel tube, which may result in fiber attenuation increases or even breakage, hence, affecting the efficiency and life expectancy of the fiber.
To provide excess length of the fiber, there must be space for the fiber to be curved. One solution is to increase the outer diameter of the FIMT. However, in the inner circular space defined by the inner diameter, increasing the outer diameter of the FIMT may cause the outer diameter of the insulating layer of the conductor to decrease. Thinning of the insulation layer causes a decrease in the insulation capacity of the conductor. The hybrid cable may not be able to supply power due to damage to the insulation of the wires in challenging environment.